A substrate produced according to this method is already frequently used in practice as a printing stencil, such as in the manufacture of electronic modules containing SMT components. Prior to the placement of these components, solder paste is forced through the substrate openings onto connection pads of a printed circuit board, using the screen-printing technique.
Dimensionally stable printing stencils, which today are mostly made of steel or nickel, or, less frequently, of polyimide, are a prerequisite for carrying out the screen-printing technique. In screen-printing, the material thickness of the substrate forming the printing stencil determines the height of the printed solder paste.
In addition to the classical SMT production, screen-printing techniques using printing stencils are used, inter alia, also in the manufacture of wafers, high pin-count chip packages, ceramic multilayers, flip-chips, and OLED's (organic light emitting diodes).
Another important field of application for such stencils or masks is the use in sputtering and vapor deposition methods.
For an optimum printing result, high demands are placed on the printing stencil in terms of accuracy and reproducibility of the position and dimensions of the openings to be formed, and with regard to a uniform stress distribution across the entire surface of the printing stencil.
The weakening of the material, which is associated with the formation of the openings in the substrate and leads to an altogether changed state of stress of the substrate fixed in position by the tensioning frame, turns out to be a problem in practice. This change in the state of stress causes an unwanted deviation in position, in particular a shift of the already formed openings relative to the tensioning frame and also relative to openings which are to be formed subsequently.
Therefore, it is common practice to initially form all desired openings at the predetermined positions in a first operation, but first with reduced dimensions. In a second operation, the openings are then enlarged to their respective desired dimensions. In this manner, the shifts occurring after the first operation can be compensated for in the second operation. In this connection, the additional time required for the second operation turns out to be a disadvantage.
One idea that has been thought of so far is to predict the changing state of stress using a model in a computer and to derive correction values therefrom, said correction values entering into the determination of the positions when forming the openings. However, this idea has turned out to be very complicated and not very promising from an economic point of view.
Furthermore, DE 100 34 648 B4 describes a method for manufacturing a printing stencil, where a metal stencil is directly and non-releasably secured to a biased frame and subsequently tensioned by removing the bias from the frame. The openings forming the print pattern can be formed in the metal stencil by punching or boring, either before or after the metal stencil is mounted on the frame. The printing stencil can be used for PCB assembly, module production, or wafer bumping.
In a method for fabricating mask configurations according to DE 101 41 497 A1, it is proposed that systematic deviations in a configuration of recesses to be produced in a mask from a desired configuration be largely prevented by carrying out the patterning of the mask substrate in a sequence of subprocesses, said subprocesses being matched such that the deviations resulting from these subprocesses cancel each other out, thus providing for error correction.
Further, U.S. 2003/0041753 A1 describes a stencil or mask, which is provided with openings.